The material constituting the electrodes of a fuel cell operating at low temperature (PEMFC, DMFC, alkaline solid membrane battery) is produced on a carbon support based on fabric, paper or felt, which plays the role of support for the diffusion layer and ensures the mechanical strength of the electrode.
The electron conducting properties of the electrode material and of the gas diffusion material derive from this support material and from the carbon inks or pastes introduced mechanically or sprayed on the surface of the support of the diffusion layer to form the charge percolation network.
FIG. 1 shows a schematic view of an assembly of an electrode of the type in question of the prior art. Microporous carbon 1 is generally sprayed on one of the faces of the diffusion layer to support the catalyst layer and to ensure good gas diffusion.
The catalyst layer is a fundamental element of the membrane-electrode assembly. Due to the presence of the catalyst particles 2 (platinum or other noble metals), the hydrogen oxidation and oxygen reduction reactions implemented in the fuel cells occur on either side of the proton conducting separating membrane 3 and thereby permit the generation of electrons.
The formulation of the catalyst layer with low loads of noble metals is one of the key factors in the development of PEMFC type fuel cell. In fact, several economic studies conducted in the early 2000 s have demonstrated that the platinum introduced in the catalyst layers and the forming thereof constituted the second most costly component in the cell. A knowledge of physicochemical and electrochemical transfers occurring in the electrode materials shows that the catalyst particle content in the cell can only be reduced by optimising the morphology of the catalyst zone.
The active layer forming methods most commonly reported in the literature are based on the principle of deposition, on the diffusion layer, of a spray of C/Pt particles placed in suspension in a light solvent, such as alcohol, and incorporating a polymer binder, the latter ensuring the mechanical strength and the water management. Over the last twenty years, a sharp decrease in platinum loads in the electrode material has been achieved.
This decrease was associated with the combination of platinum nanoparticles immobilised on carbon 5 and the use of a proton conducting impregnating film 4. Thus, the catalytic site becomes active as it integrates the proton conduction network and the electron charge percolation network directly in contact with the particle. This zone is active to the extent of the accessibility of the fuel or the oxidising gas, that is, it is limited by the material input.
In present embodiments, as illustrated in FIG. 1, the morphology of the active layer is unfortunately not optimal. In fact, only 50 to 75% of the platinum introduced into these active layers is recognised as electroactive. This loss of electroactivity is associated with the poor distribution of the catalyst. The various limitations have the following causes:                non-optimal electron charge transfer, the carbon percolation network then not being continuous from the C/Pt particle to the diffusion layer and through the diffusion layer to the bipolar plates;        or by limiting material transport, the catalyst particle no longer being reached by the gas (if for example the platinum is located opposite the carbon particle);        or by a proton conduction network (obtained by impregnation with a proton conducting solution or by contact of the electrode with the membrane) preventing access to the catalyst particle.        
Optimal formulations of active layers on diffusion layer support have been described. More particularly, the formulation of E-Tek electrodes, sold by DeNora, is reported in documents EP-A-0 872 906 and EP-A-0 928 036.
The literature reports the possibility of improving the performance of the cells by adjusting the methods of incorporation of the proton conductor. By optimising the Nafion content incorporated in the active layer, the kinetic operating range of the fuel cell (low current density range) can be improved. However, for very low platinum loads, this improvement occurs to the detriment of the high current density range, with a limitation by mass transfer being reached more rapidly and an increase in electrode-membrane interface resistance.
The active layers have been the subject of several modelings aimed to determine their optimal organisation, by increasing the geometric surface area of the platinum developed and by minimising the resistance effects associated with the proton conductor and the carbon. Based on these modelings, new structures have been tested, either introducing multilayer structures (alternating catalyst layer and proton conducting film), or fibres impregnated with proton conductor, or porophores. The most satisfactory results were obtained by the introduction of porophoric systems into the active layers, as described in document US 2001/0031389. The mass transfer is improved, thereby serving to meet the demand for operating applications of the cells in air.
In conjunction with the idea of a more open active layer to avoid limiting material transport and to reinforce the electron charge percolation network, the possibility has been developed of directly immobilising the catalyst particles on the diffusion layer support.
This type of operation is reported in the literature with various techniques: pulsed electrodeposition, microemulsion, spray coating, vacuum deposition (EBPVD in document U.S. Pat. No. 6,610,436, CCVD described in document WO 03/015199). However, these various techniques have non-negligible drawbacks.
Microemulsion processes are not suitable for obtaining a controlled particle distribution if directly deposited on the diffusion layer.
In the case of deposits by electrodeposition, the particle size obtained is generally higher than 50 nm and therefore has low electroactivity.
For deposits by PVD, the limitation to this type of process resides in the difficulty of obtaining nanodispersions of catalyst nanoparticles and of locating the catalysts close to the membrane, without losing some depth in order to respond to a higher catalytic activity during power draws.
The ion-beam processes, as described in document U.S. Pat. No. 6,673,127, do not allow a dispersion in depth, in more than 5 nm of the thickness of the diffusion layer.
The standard CVD deposition process has a particle growth yield that is too low for the deposition temperatures required by electrode supports (T<350° C.), or requires the use of alycyclic platinum precursors (see document U.S. Pat. No. 6,162,712) which are unfortunately very unstable at high temperature and in air.
It therefore clearly appears that the catalyst layers currently present in PEMFC type fuel cells, shown in FIG. 1, have the drawback of immobilising a high catalyst load which remains inaccessible to the proton conduction and gas diffusion network, or blocks the electron conduction.
An obvious need therefore exists to obtain novel catalyst layer structures not having all the above drawbacks and hence to identify a deposition technology suitable for producing such structures.